Conventionally, as for an AlGaN/GaN HJFET structure, there has been a report of an MISFET structure in which an insulation film is formed between a source and a drain and then a gate electrode is formed thereon, as it can reduce a gate leak current and increase a positively applied gate bias to render a high current available, and so its application to a high-output use is expected.
For instance, Adivarahan V. et al. reported the structure employing a Si3N4 film as the insulation film on AlGaN/GaN in IEEE Electron Device Letters, Vol. 24, No. 9, pp. 541 (2003).
FIG. 6 is a sectional view illustrating the structure of a field-effect transistor reported by Adivarahan V. et al. As shown in FIG. 6, Ti/Al/Ti/Au is deposited in layered form as a source electrode 1005 and a drain electrode 1006 after sequentially layering up an AlN layer 1002 of 50-nm film thickness, a GaN layer 1003 of 1.5-μm film thickness and an AlGaN layer 1004 of 25-nm film thickness on a 4H-SiC substrate 1001, and then it is subjected to heat treatment at 850° C. for 1 minute.
Furthermore, after a Si3N4 film 1007 is laminated over, a gate electrode 1008 made of Ni/Au is formed thereon, and then a protective film 1009 is formed with the Si3N4 film lastly. Thus, it is possible, by inserting the insulation film between a semiconductor layer and the gate electrode so as to form the MISFET structure, to reduce the gate leak current and increase the positively applicable gate bias, whereby high output operation is attained.